Organic insecticide compounds are often applied in the form of a dilute aqueous composition in order to achieve a good interaction with the target organisms. However, most active ingredients that are used as insecticides, in particular pyrethroid compounds, are only sparingly or even insoluble in water, i.e. they usually have a water-solubility of not more than 5 g/l, often not more than 1 g/l and particularly not more than 0.1 g/l at 25° C./1013 mbar. Therefore, formulators are often confronted with difficulties in formulating pesticide compounds in stable formulations that can be easily diluted with water.
Organic insecticides having a limited solubility in water are often formulated as aqueous suspension concentrate (SC) which can be diluted with water for use in the field. Suspension concentrates are formulations wherein the active ingredient is present in the form of finely divided solid particles, which are suspended in an aqueous dispersing medium utilizing surface-active compounds (surfactants), such as wetting agents, dispersants and rheological or suspending aids for stabilising the active ingredient particles in the dispersing medium. However, problems are often encountered with SC's as a result of settling during prolonged storage or storage at elevated temperatures, the resistance of settled particles to re-suspension and the formation of crystalline material upon storage. As a consequence, the formulations are difficult to handle and the bioefficacy may be inconsistent. Moreover, SC's are limited to actives that have a relatively high melting point. Most agrochemicals are sparingly water-soluble and become partly “deactivated” with water when formulated as an aqueous SC. Therefore, it is highly desirable to formulate these compounds as an oil in water microemulsion, hereinafter also referred to as ME or ME formulations.
Microemulsions are multiphase systems comprising a disperse phase and a continuous phase. In contrast to macro emulsions, the average particle (droplets) size (Z=average diameter as determined by light scattering) of the disperse phase in microemulsions is at least 5 times smaller than in macro emulsions and generally does not exceed 200 nm, while the average diameter of the droplets in macro emulsions is in μm range. Microemulsions are known as bicontinuous structures with intricate channels of oily and aqueous phases. Due to the small particle size (droplet size) of the disperse phase, or the intricate channels, microemulsions have a translucent appearance.
ME formulation of organic insecticide compounds are usually water based and additionally contain at least one surfactant and at least one cosolvent or cosurfactant, which is usually an organic solvent or a low molecular weight polyalkylene ether. By using ME formulations risks such as inflammability and toxicity, environmental concerns and costs are reduced in comparison with emulsifiable concentrate (EC) techniques, because water is the main constituent. Due to the small particle size of the disperse phase containing the active ingredient, an increase in bioavailability can often be achieved. However, it is difficult to maintain the stability of ME formulation of active ingredients having a low water-solubility with respect to the droplet size and uniformity and crystallization of active ingredient that may occur. Moreover, it is also difficult to maintain the droplet size stability when the ME formulation is diluted with water. However, a stable droplet size after dilution, i.e. maintaining a small droplet size, is important to achieve preferable biological activities. Therefore, much effort has been made in order to develop stable water-based microemulsion formulation. The aforementioned problems are particularly pronounced in case of pyrethroid compounds.
WO 90/06681 describes microemulsion formulations containing 0.01 to 1.0% of a solution containing one or more synthetic pyrethroid(s), C2-C3-alkoxylated nonylphenol, linear calcium dodecylbenzene sulfonate, polyoxyethylene sorbitan monolaurate and a mixture of organic solvents, and water ad 100%.
WO 99/66300 describes an aqueous microemulsion of an agriculturally active pyrethroid which comprises N-alkylated pyrrolidones, a EO/PO block copolymer surfactant, an ethoxylated castor oil or a tristyryl phenol ethoxylate, a phosphate ester as pH buffer and at least 80% of water.
EP 0160182 describes aqueous-based microemulsion compositions containing a synthetic pyrethroid, a surfactant blend, i.e. calcium dodecylbenzene sulfonate, ethoxylated distyrylphenol ammonium sulfate and ethoxylated tristyrylphenol, and adjuvants, such as antifoamers, anti-freezing agents, thickeners and preservaties.
The microemulsions disclosed in prior art are solely suited for ULV application and not stable upon dilution with water. There is an ongoing need for ME formulations of organic insecticide compounds which are physically and chemically stable, i.e. upon storage no phase separation should be observed and the insecticide compound should not undergo remarkable degradation or suffer from inactivation. The ME formulations should have an intricate and stable structure. In particular, the formulation should be stable upon dilution with water, i.e. they should provide stable size distribution of small droplets after dilution with water. Moreover, they should provide reduced tendency of the active ingredient to crystallize, in particular after dilution of the formulation with water. Moreover the ME formulation should maintain its liquid monophasic state in a broad range of temperatures, i.e. in a range of from −10 to at least 50° C. In particular ME formulations are required which provide stable formulations of organic insecticides, in particular pyrethroid compounds, preferably pyrethroid esters having a biphenyl ether moiety and especially flucythrinate or alpha cypermethrin or mixtures thereof.